308 THE EVOLUTION OF PETROLOGICAL IDEAS. 



refusion of homogeneous rocks niuy give rise to ;i heterogeneous 

 inagniii, comparable to that produced by Faraday in his experiments 

 on glass. The cause of some of the variations^ in igneous rocks is 

 therefore probably to be sought for in actions which antedate the 

 formatioi\ of the magmas. But even homogeneous magmas may 

 become moditied b}^ the al)soi'ption or assimilation of the rocks 

 through which the}' pass. This point has been clearly established and 

 especially emphasized by M. Michel Levy, Professor Barrois, and 

 Professor Lacroix in France, and by Dr. Johnston Lavis, Professor 

 Sollas, Professor Cole, and Mr. Harker in this country. 



That it is a vera causa is admitted on all hands, but ditierences of 

 opinion exist as to the extent to which it should be applied in explain- 

 ing the origin of petrographical species. 



If we stud}'^ igneous rocks which have appeared at the surface as 

 lavas, or have been intruded at moderate depths as dykes, sills, 

 laccolites, or bosses, the evidence of absorption is, in m}' judgment, 

 so slight as to be practicall}' negligible; but if we pass from such 

 regions to others in which plutonic rocks are found in relation with 

 crystalline schists and study 'Mes appareils granitiques a racines 

 prof ondes " of M. Michel Levy, the case is different. It may be that 

 the final solution of the problem of the origin of igneous magmas will 

 be found in these regions; but here we touch a question which belongs 

 to the future rather than to the past, and lies, therefore, beyond the 

 scope of this address. So far as I am concerned, I will confess that 

 my ideas are not tixed. At present I am not disposed to attach much 

 importance to theories involving differentiation in situ by unaided 

 molecular flow in dikes and laccolites; but rather to attri))ute such 

 variation as does occur to successive eruptions, or to a continuous 

 change in the nature of the material during the process of intrusion. 

 The great difficulty in applying any theory that involves ditt"er(Mitia- 

 tion in situ to such cases arises from the slight effect of the igneous 

 magmas on the containing walls — a fact which negatives the idea that 

 the material arrived at the place where we now hnd it in a condition 

 of superfusion, or that it remained fluid long enough to enal)le any 

 considerable difl'usion to take place. 



Qui- ideas as to the origin of igneous rocks are still "en pleine 

 evolution," Conditions are rapidly changing in consequence of dis- 

 coveries in geology and physical chemistry. Rival theories are strug- 

 gling for existence, and although it is safe to predict that some will 

 become extinct, that others will be modifled, and that natural selection 

 will finally bring about the survival of the fittest, it is impossi))le to 

 determine at ])rcsent tli(^ r(^lativ(> inij)ortanc(> of those wliicli claim 

 oui" attention. 



The origin of ])etrographical species, so far as the igneous rocks 

 are concerned, is a problem the final solution of which has been 

 handed on by the nineteenth century to its successor. 



